Even if you think that LAN miles are worthless, you'd be paying $500 for flying Star Alliance vs. LAN, or 30% more, and that's a lot in my opinion. Can you credit LAN flights to AA and use the miles for a one-way award someplace?

Don't buy the ticket from UA. Judging from your Silver status, I figure you don't fly a whole lot and this is probably the most expensive ticket you'll buy in the near future. Use the miles for it and save 1600 dollars that you can use for anything.

Also, for 1600 bucks, you can flat out buy your miles back from US Airways if you decide you'd like the miles more than the cash in the future. But you can't convert miles back to cash...

Good luck.